Teacher's notes and answers to questions in the book - Hodder Plus ...

WJEC GCSE Additional Science Teacher’s Notes
_ What is osmosis (pages 29–32)_________________
Questions
3. Here are two answers given in an exam. Neither of them got any marks. What is wrong with each of
these answers
a ‘In osmosis, water travels in the opposite way from diffusion, that is, from a dilute solution to a
concentrated solution.’
There is more water in a dilute solution than in a concentrated solution, so the direction of
water movement in osmosis is still down a concentration gradient. (It is best to avoid
talking about a high ‘concentration’ of water, as concentrations should be expressed in
terms of the solute, not the solvent.)
b ‘When the concentrations inside and outside a cell are equal, the movement of water stops.’
The movement of water does not stop, it just balances out or ‘reaches an equilibrium’. The
net movement of water stops.
PRACTICAL Osmosis in potatoes (pages 31–32)
1. Describe the trend seen in your results, and explain how it was caused.
As the concentration of sugar increases, the weight increase gets less, and eventually
becomes a weight loss, which then increases. It is important to refer to both the decrease
in weight gain and the increase in weight loss. This is caused initially by the decrease in
the concentration gradient between the solution (high water) and the potato (low water) and
then by the increase in concentration gradient between the potato (high water) and the
solution (low water).
Explaining this is quite a challenge in terms of communication skills and it is
important to know that pupils are expected to provide a full and clear explanation to
gain credit.
2. The experiment can also be done by measuring change in length of the potato cylinders. Explain
why measuring mass is better.
Mass takes account of an increase in all dimensions. It would be possible (though unlikely)
for the cylinder to increase in diameter but not length, which would lead to inaccurate
results.
3. Why was it important to blot the potato cylinders dry before weighing them in step 9
You don’t want to weigh liquid which is not actually inside the potato. This would lead to
inaccurate results.
4. Why were you asked to record % change in mass and plot that in the graph rather than just change
in mass
The potato cylinders were identical in size but not necessarily in mass. % increase takes
account of different starting weights.
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